1. Field of the Invention
The present invention generally relates to a method of depositing thin films by plasma-enhanced chemical vapor deposition (CVD). In particular, the invention relates to a method of depositing thin films whereby a thin film is deposited on a substrate surface using a plasma-enhanced CVD reaction in the production process of a semiconductor device.
2. Description of the Related Art
Progress has been made in recent years in semiconductor processing by increasing the degree of integration of the circuit elements and by producing finer features in semiconductor devices. However, achieving finer circuit elements requires novel techniques, e.g., for embedding adequate films in fine holes (contact holes and via holes), for reducing the size of the step formed in circuit elements, and for preventing the breaking of wiring due to electro-migration or heating as a result of increased current density. New techniques have been developed for certain processes, e.g., for the deposition of blanket tungsten films (B-W films) by a CVD method and for the deposition of Al (aluminum) films by a sputtering method.
When depositing B-W films and Al films, a Ti (titanium) film and a TiN (titanium nitride) film are deposited in the case of a contact hole, for example, between the conducting film and the underlying layer in order to ensure conductivity with the underlying layer, to ensure adhesion, and to prevent inter-diffusion (to ensure barrier properties). In the case of elements with wiring line widths ranging from 1.0 to 0.25 .mu.m, a sputtering method is used for depositing the Ti film and the TiN film. However, in the case of elements with wiring line widths ranging from 0.25 to 0.1 .mu.m, it is difficult to obtain satisfactory step coverage with a sputtering method. As a result, a plasma-enhanced CVD method whereby a thin film is deposited by a gas phase growth using a plasma-enhanced chemical reaction in the vicinity of the substrate surface has been used.
To deposit a Ti film using the plasma-enhanced CVD method, a plasma is generated with hydrogen (H.sub.2) gas which has been introduced beforehand in a reactor, a reaction gas containing titanium tetrachloride (TiCl.sub.4) and H.sub.2 is then introduced into the reactor, the TiCl.sub.4 or a precursor produced by degradation of the TiCl.sub.4 is reduced by the active hydrogen ions and atoms generated by the plasma, and a Ti film is deposited on the substrate. Similarly, to deposit a TiN film using the plasma-enhanced CVD method, a plasma is generated with a gaseous mixture of nitrogen (N.sub.2) and hydrogen (H.sub.2), a reaction gas mixture of TiCl.sub.4, N.sub.2, and H.sub.2 is then introduced into the reactor, the TiCl.sub.4 or precursor is nitrided by the active nitrogen ions and atoms generated by the plasma, and a TiN film is deposited on the substrate. Titanium tetrachloride (TiCl.sub.4) is used as the reactive gas in the methods described above because the surface of the deposited film is smooth, and the step coverage is excellent.
Typical apparatus used for depositing such thin films include either a parallel plate type plasma-enhanced CVD apparatus which uses a radio frequency (RF) of 13.56 MHz to generate the plasma (see, e.g., N. J. Lanno et al., J. Electrochem. Soc., 136 (1989), p.276) or an electron cyclotron resonance (ECR) type plasma-enhanced CVD apparatus which produces a high density plasma (see, e.g., T. Akahori et al. J. Appl. Phys., 30 (1991), p.3558; and T. Miyamoto, Proceedings of the VLSI Multilevel Interconnection Conference, (1995), p.195).
However, the conventional plasma-enhanced CVD techniques described above have certain drawbacks. For example, when a parallel plate type plasma-enhanced CVD apparatus having a frequency of 13.56 MHz is employed, chlorine (Cl) from the TiCl.sub.4 reaction gas remains in the deposited Ti film or TiN film. This residual chlorine corrodes the Al wiring film. In addition, a lot of undegraded TiCl.sub.4 remains in the plasma. The undegraded TiCl.sub.4 erodes the Si in the underlayer at the bottom of a contact hole. The erosion of the Si underlayer is a problem in that it decreases the surface smoothness of the Ti film or TiN film, and lowers the reliability of the circuit elements.
When an ECR type plasma-enhanced CVD apparatus is used, a high density plasma is obtained. As a result, there is less undegraded TiCl.sub.4. Additionally, the amount of Cl in the Ti film or TiN film is lower. This is because some of the Cl from the TiCl.sub.4 reaction gas is removed by the active hydrogen ions and atoms which are produced in greater quantities. However, not enough of the Cl is removed. Moreover, the Ti film and TiN film step coverage properties are inadequate when using an ECR type plasma-enhanced CVD apparatus. Consequently, the barrier properties cannot be ensured satisfactorily, and the reliability of the elements is reduced.
The aforementioned problems have been outlined by particular reference to contact holes, but the same can be said with other types of fine hole such as via holes. Via holes are formed in an SiO.sub.2 layer which is formed over a metal layer. In the case of a via hole, only a TiN film is deposited on the underlayer so that only the problems associated with TiN film deposition are of concern.
It is an object of the present invention to solve the problems associated with conventional plasma-enhanced CVD processes mentioned above. In particular, it is an object of the present invention to provide a method for depositing thin films, e.g., Ti films or TiN films, by plasma-enhanced CVD in which the amount of chlorine remaining in the Ti film or TiN film is low, the erosion of the underlayer is minimized, the surface of the deposited film is smooth, the step coverage properties are good, the production yield is increased, and the reliability of the elements is increased. As will be readily apparent from the description below, the present invention achieves these and other objects.